A tire such as this, generally intended to bear heavy loads, comprises a radial carcass reinforcement, and a crown reinforcement made up of at least two working crown plies, formed of inextensible reinforcing elements which, from one ply to the next, crossed one another making equal or unequal angles of between 10 and 45° with the circumferential direction.
The crown reinforcements of radial tires, especially as far as very large sized tires are concerned, are subjected to substantial deformations which give rise, between the edges of the crossed plies, to longitudinal and transverse shear stresses (the longitudinal shear is greater than the transverse shear when the cords of the crossed plies are at small angles to the circumferential direction), together with a delamination stress, a radial stress that has a tendency to separate the edges of the two plies radially. The said stresses are due primarily to the tire inflation pressure which means that the pressure known as the hoop pressure between the carcass reinforcement and the crown reinforcement tends to cause the said crown reinforcement to expand circumferentially. The said stresses are also due to the load borne by the tire under driving conditions, with the creation of a contact patch at the region of contact between the ground and the tire. The said stresses are also due to cornering on the tire during driving. These stresses generate cracks in the rubber compound adjacent to the end of the shortest ply, which cracks spread into the said compound to the detriment of the endurance of a crown reinforcement and therefore of the tire.
A marked improvement in the endurance has been afforded by the use, in the crown reinforcement, of at least one protective crown ply that has an axial width greater than the width of the axially widest working ply.
What “axial” means is a direction parallel to the axis of rotation of the tire and what “radial” means is a direction that intersects the axis of rotation of the tire and is perpendicular thereto. The axis of rotation of the tire is the axis about which it rotates in normal use. The circumferential mid-plane is a plane perpendicular to the axis of rotation of the tire and which divides the tire into two halves. A radial plane is a plane which contains the axis of rotation of the tire.
Another solution, as described in patent FR 2 421 742, is to distribute the stresses that as the tire undergoes cornering generate separation between working crown plies more favorably, by increasing the number of working plies.
Increasing the number of working plies is not without its disadvantages, particularly at the centre of the crown reinforcement where the number of plies has a direct impact on the bending rigidity of the crown of the tire. As this rigidity increases, it follows that impacts suffered by the crown of the tire, when driving over large stones for example, may lead to irreparable tire damage, as a result of this increased rigidity.
Patent application WO 00/54992 has also proposed, in order to avoid this disadvantage, creating a working crown reinforcement consisting of at least three continuous working plies and at least one half-ply, on each side of the circumferential mid-plane, positioned between the edges of at least two radially adjacent continuous working plies and the special feature of which is, in particular, that it makes an angle greater than 25° and between 5° and 15° greater than the largest of the angles of the continuous working plies, with the circumferential direction. The results obtained with this type of architecture have proved entirely satisfactory for the tire sizes tested, running over normal terrain.